What's the difference between i.e. and e.g.?
late 14c., "either of the two circles in the celestial sphere which describe the northernmost and southernmost points of the ecliptic," from Late Latin tropicus "of or pertaining to the solstice" (as a noun, "one of the tropics"), from Latin tropicus "pertaining to a turn," from Greek tropikos "of or pertaining to a turn or change, or to the solstice" (as a noun, "the solstice"), from trope "a turning" (see trope).
The notion is of the point at which the sun "turns back" after reaching its northernmost or southernmost point in the sky. Extended 1520s to the corresponding latitudes on the earth's surface (23 degrees 28 minutes north and south); meaning "region between these parallels" is from 1837. Tropical "hot and lush like the climate of the tropics" is first attested 1834.